This invention relates to an output circuit for feeding a current to an electromagnet, an electromagnetic actuator, or a linear motor.
Furthermore, this invention relates to a brushless motor usable for a tape recorder, a record player, a video tape recorder, etc.
A drive circuit of this type is well-known and includes an output transistor, a current detecting resistor, and a comparator to keep the output transistor non-saturated. The output transistor is a current control transistor, whose emitter current is detected by a current detecting resistor. An error between the current command signal and the current detecting signal is amplified by a differential amplifier to drive the base of transistor, thereby allowing the emitter current to flow up the current command signal. The collector of the transistor connects with a load, and the transistor and amplifier control voltage of load at the power source side so that the collector voltage follows up the reference supply voltage in order to save power consumption of transistor and avoid saturation of transistor, the reference supply voltage is set to a proper value, whereby an operating point where the transistor is not saturated.
Furthermore, the output circuit in the prior art has been used for a conventional brushless motor. Next, explanation will be given on an example of conventional brushless motor which switches a 3-phase armature current by the transistor and controls a generated torque by a command input.
The rotational positions of multipolar-magnetized permanent magnet rotor and windings L.sub.1 to L.sub.3 of 3-phase armature windings are detected by a position detector and transmitted to two position-signal-switching circuits. The position signal switching circuits each are of 3-differential construction and connected to the base of each transistor in a group of two sets of output transistors different in electric conductivity and the collectors thereof correspond to each other, the group of two sets of transistors having the emitters connected in common and the collectors which are connected at the phases corresponding to each other so as to be of push-pull construction and which are connected to the corresponding phases at the armature windings respectively. The common emitter at one group of output transistors is connected to the power source and that at the other group of the same is grounded through a resistor. Voltage at the terminal connecting with no resistor is applied into one input for a current output type differential amplifier and torque command voltage is applied to the output of differential amplifier, and an output thereof is applied to a first position signal switching circuit of current mirror system. The other ends of armature windings are connected in common and connected to one input of second differential amplifier, the other input thereof being applied with a half of supply voltage by a voltage divider. The output of second differential amplifier is applied in a current mirror system to the first position signal switching circuit. The armature windings are controlled by torque command voltage, whereby torque generated by the motor is controlled by the torque command voltage. On the other hand, the common node of windings L.sub.1, L.sub.2 and L.sub.3 is kept in a half of supply voltage by a negative feedback circuit according to the second differential amplifier. Hence, potential of armature winding changes around a value of half of supply voltage and the two sets of transistor groups operate at about equal emitter-collector voltage, whereby in case where the torque command voltage or the number of rotations of motor increase to increase voltage across both ends of the respective windings L.sub.1 to L.sub.3, the two sets of transistor groups will be saturated to an about equal extent to thereby improve the utilization factor.
The supply voltage is set high enough not to saturate the two sets of transistor groups in order to obtain the generated torque necessary for the start and rotation of motor. Here, between the collector and the emitter is applied voltage of the remainder of substracting from the supply voltage the sum of voltage drop of resistor, voltage drop by DC resistance of windings L.sub.1 to L.sub.3, and counter-generated voltage of windings L.sub.1 to L.sub.3 accompanied by rotations thereof. However, when the armature current is small or the number of rotations of motor is small, voltages of transistor group, collector and emitter, become high and power consumption of each transistor group increases, thereby causing the inconvenience of large power loss.
Also, in a case where the supply voltage is lowered, either one of transistor groups is saturated, at which time the base current flows also in the transistor inherently to be non-conductive so that an armature current harmful to the torque generated by motor flows in the transistor, thereby creating the inconvenience of generating a torque loss or abnormal vibrations.
In order to eliminate the above inconvenience, the supply voltage is controlled to allow the collector voltage at the transistor group to follow the reference voltage.
Such prior art, however, has a serious defect such that voltage across the collector and emitter of transistor not-saturated changes due to a load current, a temperature of transistor, or variation in transistors, whereby the reference voltage should be set higher, or follow the factor of variation. When the reference voltage is set higher, V.sub.CE of transistor becomes high to increase power consumption. Also, the circuit construction becomes complicated for allowing the reference voltage to follow the factor of variation, which leads to a high manufacturing cost, thereby being industrially disadvantageous.
The above has been a grave defect with respect to miniaturization and power consumption saving for the machinary utilizing the brushless motor.